1. Field of the Invention
This invention relates to apparatus and methods for winding strips of filament reinforced thermoplastic onto rotating objects in order to produce wound structures. More particularly, the invention relates to apparatus and methods for simultaneously winding a plurality of filament reinforced thermoplastic strips onto a rotating object, such as a mandrel.
2. Description of the Prior Art
Fiber or filament reinforced polymers, including thermosets and thermoplastics, are known for their high strength, light weight, corrosion resistance, and ease of fabrication compared to the wood or metal structures which they commonly replace. Because of these qualities, the filament reinforced thermoset and thermoplastic structures are in increasing demand by many industries, particularly the aerospace industry.
The winding of a strip of polymer, plastic, rubber, and other materials to make cylindrical articles is well known. Particularly such use of thermoset polymer composites, including polyester, vinylester, and epoxies, is well known. Pipe, hose, and storage tanks, particularly underground storage tanks, are articles commonly made by such winding. It is also known to construct such cylindrical structures by winding a filament reinforced thermoplastic strip in multiple layers upon a rotating form or mandrel. This is commonly known as filament winding and the apparatus is commonly called a filament winding machine. One type of filament winding apparatus and method, as well as typical materials used therewith, is described in U.S. Pat. No. 4,078,957, issued to Bradt, which is incorporated herein by reference thereto.
Normally, the strip is moved laterally with respect to the longitudinal axis of the mandrel as the mandrel rotates in order to evenly distribute the strips in a layer along the length of the mandrel. The strips are normally helically wound on the mandrel with adjacent layers wound in opposing or crossed helixes. It is usually desired to distribute the strips in contiguous juxtaposition, i.e., in side to side contact, in order to form a continuous structure.
The filament reinforced thermoplastic strip may be produced immediately prior to being wound on the mandrel by a well known process, such as the wet impregnation process. In the wet impregnation process, a plurality of fibers or filaments are pulled from a creel and through a resin bath for impregnation of the filaments with the resin. The resin impregnated filaments, commonly referred to as "prepreg feed material" are pulled through a heated forming zone to form the filament reinforced thermoplastic strip to be wound about the mandrel.
More typically, the filament reinforced thermoplastic strip is purchased or provided to the filament winding apparatus ready-made. Such ready-made filament reinforced thermoplastic strips are commonly known as preimpregnated strips, prepregs, tow pregs, pregs, and other similar names.
Filament reinforced thermoplastic strips may be wound to create structures of various sizes and shapes. In many such structures, the width of the strip may need to be closely controlled in order to produce the desired shape of wound structure in an acceptable quality. For example, in a wound structure of large size, it may be desirable to use wide strips in order to speed the winding process. It is common to need a width of strip which is not readily available in a single premanufactured "prepreg" strip. In such a situation, it would be of great advantage to be able to simultaneously wind multiple strips which in combination add to the desired overall strip width.
It is also common to have a single cylindrical shape which has areas of various diameters, continuously increasing or decreasing diameters, etc. In such structures, because of the helical pattern of the strip, one side of the strip must cover more wound structure surface area, i.e., the side of the strip which is being applied to the area of wound structure having larger diameter must cover more structure. If one strip of desired width is used, wrinkling or strip damage may occur because of the change in mandrel, or wound structure, diameter and tension across the width of the strip. In such a situation, it would be of great advantage to use simultaneously applied multiple strips which are free to travel at differing speeds and distances in order to accommodate the changing diameter of the mandrel.
However, the industry has met with limited success in simultaneously winding multiple plastic strips. A major problem encountered when constructing filament reinforced thermoplastic structures by simultaneously winding a plurality of strips upon a rotating mandrel, is that the strips separate and create gaps or voids in the layer of the structure being created. This problem is worse if the mandrel is large, e.g., greater than four inches in diameter, or if the combined width of the simultaneously applied strips is relatively narrow, because the lateral motion of the simultaneously applied strips with respect to the longitudinal axis of the mandrel is slow. (Normally, the mandrel and strip supply means are adjusted so that the strip advances along the longitudinal axis of the mandrel with each rotation of the mandrel.) This slow lateral motion increases the tendency of the simultaneously applied strips to separate and create gaps or voids in the layer of the structure being wound. These gaps or voids also weaken the structure and create a need to either increase the number of layers or otherwise compensate for such gaps and voids.
The present invention is directed to apparatus and methods which prevent the separation of filament reinforced thermoplastic strips which are simultaneously applied to a rotating mandrel or similar object.